Our long term goal is to understand the mechanisms underlying the modulation of neurotransmitter release from presynaptic nerve terminal, an important form of synaptic plasticity involved in learning and memory. Changes in synaptic function are also likely to be important cellular loci for both neurologic and psychiatric disorders. Here, we focus our attention on the role that Ca++ channel modulation plays in presynaptic inhibition in sensory neurons of the marine snail Aplysia in response to the neuropeptide FMRFamide. Presynaptic inhibition is an important mechanism for regulating synaptic transmission. In vertebrate sensory neurons, presynaptic inhibition has been postulated to play a role in modulating the perception of pain and other stimuli. In Aplysia sensory neurons, presynaptic inhibition produced by FMRFamide (a close relative of the opioid peptides) leads to behavioral inhibition. Like presynaptic inhibition in vertebrates, the presynaptic inhibition in Aplysia is associated with an increase in K+ current and a decrease in Ca2+ current. What are the biochemical mechanisms that mediate presynaptic inhibition? In Aplysia, we previously showed that an increase in K+ channel opening with FMRFamide was mediated by the 12-lipoxygenase metabolites of arachidonic acid, a novel second messenger pathway for neurons. What is the second messenger mechanism underlying inhibition of the calcium current? In vertebrates, activation of protein kinase C appears to be involved in some, but perhaps not all, cases of modulatory decreases in calcium current. In Aplysia sensory neurons, protein kinase C does not appear to be involved. Is the decrease in calcium current in Aplysia also mediated by arachidonic acid metabolites? Are other second messenger pathways involved? Does Ca2+ channel inhibition involve the parallel action of multiple second messenger pathways or might the different cascades act in series? If arachidonic acid metabolism is involved, which metabolite is implicated. Does the same metabolite modulate both K+ channel opening and calcium current inhibition or do different branches of the arachidonic acid cascade serve different modulatory functions? Whole cell calcium current and single calcium channel currents will be studied using the patch clamp technique in Aplysia sensory neurons. The effects of FMRFamide on calcium current will be compared to the action of exogenously applied arachidonic acid and arachidonate metabolites. The effects of various inhibitors of the arachidonic acid cascade will also be tested. Finally, possible roles for other modulatory pathways will be investigated.